1. Technical Field
This disclosure relates to semiconductor fabrication and more particularly, to a method and apparatus for an improved interface between metal lines by providing a metal cap.
2. Description of the Related Art
Contact yield to Aluminum (Al) dual damascene levels is often limited by Al compounds, such as AlOx, which are created at the bottom of a via as a combination of subsequent process steps on the chemically unstable Al surface left by a chemical mechanical polish (CMP) process. These process steps typically include insulator deposition, reactive ion etch (RIE) during the via (over)etch, resist ashing, and chemical wet cleans. These compounds and/or residuals are also hard to remove in a pre-metal deposition sputter clean which often results in bad yields. Another problem with damascene structures is that there is no redundant conductor on top of the Al lines; the lack of a redundant conductor may degrade the reliability of the interconnect.
Back-end-of-line (BEOL) metallization schemes to realize interconnects in integrated circuits (ICs) can be realized by subtractive methods or damascene methods. In subtractive methods, a blanket metal film is patterned via photo lithography and etching. The metal film is usually a stack consisting of a low-resistivity material, such as aluminum sandwiched between barrier and liner metals such as titanium, titanium nitride or a combination of both. The resulting spaces between the lines are later filled with a dielectric.
In the case of damascene methods, a dielectric is deposited first as a blanket film. Trenches are then generated via lithography and etching techniques. These are filled with metal, again usually a stack of different types, which is then polished back to the surface of the dielectric (CMP). Typically, some overpolish is performed to ensure that there are no shorts through remaining metal between the lines. The damascene process can be easily broadened to a dual damascene process by subsequently forming vias to a lower level and trenches to hold an interconnect of a level to be realized. The vias and trenches are formed in a same blanket dielectric film prior to metal deposition and CMP processes. Dual damascene processes are typically less expensive due to the reduction of process steps. In addition, as ground rules continue to shrink, it is easier to etch dielectrics, such as, silicon oxide than it is to etch metal stacks.
Another difference between a damascene and a subtractive metallization method is that the damascene method leaves a polished surface of Al or AlCu behind which is far more reactive than Ti or TiN. This lack of a redundant conductor may degrade the reliability of the interconnect.
Because of the fact that a damascene scheme ends with a CMP step which leaves a rather reactive surface behind as described above, the next contact level needs to form a good and reliable interface with this critical surface. This is independent of the type of interconnect chosen, whether it be another dual damascene level, a stud process, or a tapered via process. The quality of the interface will depend strongly on the local composition of the polished surface. The smaller the vias get, the larger the local variations become due to effects like, for example, copper (Cu) segregation or different degrees of local oxidation. Residuals or compounds formed in the via etch process may also contribute to these effects. These effects are difficult to control or eliminate with cleaning steps.
Referring to FIG. 1, a dielectric layer 12 is patterned to form a via 14 and a trench 16. Via 14 and trench are filled with a metal, such as Al. Another dielectric layer 18 is deposited and patterned to form a via 20. During patterning of via 20, a portion 22 of Al oxidizes. This oxidation is particularly difficult to clean. When another conductive material 26, such as Al, is deposited, the interface between material 26 and an interconnect 24 includes an oxide therebetween which increases resistance, reduces yield and decreases reliability.
Therefore, a need exists for a more reliable and better controlled interface between levels of metal connections.